1. Field of the Invention
The present invention generally relates to radio frequency power amplifiers and, more particularly, to applying adaptive signal processing to control the power amplifier to improve the overall amplifier linearity and efficiency.
2. Background Information
Present second generation cellular systems, personal communications, and planned third generation wireless systems employ digital modulation techniques such as quadrature phase-shift keying (QPSK). Some QPSK systems require that the power amplifier contained in the transmitter of the portable radio handset meet strict linearity requirements which stem from the modulation format and the selected baseband filtering. The amplifier must be linear to accurately reproduce both the time varying amplitude and phase characteristics of the signal in order to minimize interference in adjacent or alternate channels and preserve the quality of the transmitted signal.
In addition to linearity, a primary consideration for power amplifiers, particularly battery powered mobile radios where extending battery life is crucial, is efficiency. Efficiency is defined as the ratio of radio frequency (RF) power output by the amplifier and supplied to the load divided by direct current (DC) power supplied to the amplifier from a power source. In general, linear power amplifiers are less efficient than non-linear power amplifiers. The lower efficiency is due in large part to the degree to which the active device operates in gain compression. By their nature, linear amplifiers only reproduce the desired amplitude and phase characteristics of the modulated input signal if operated below, or at worst, slightly in gain compression. Such operation results in an inherent trade-off between efficiency, which is maximum under gain compression, and linearity, where no gain compression occurs.
A further characteristic of second generation cellular systems which affects power amplifier efficiency and battery life of the mobile radio is the use of transmitter power control. Power control occurs when the base station commands the mobile radio to output power levels ranging from a maximum level to relatively low levels, such as 10""s of decibels (Db) from maximum. The time period over which the mobile radio functions at reduced transmit power levels is statistical in nature and dependent on numerous factors. For example, studies on selected systems have shown utilization of aggressive power control, thus placing the mobile radio transmitter in back-off mode for extended time intervals. Consequently, the radio frequency amplifier operates for extended time periods in deep back-off or very low power where efficiency may be very low. Other systems tend to operate over extended periods of time close to maximum output power. Overall efficiency depends on the amount of time that the amplifier operates at a particular power level and the corresponding efficiency at that power level.
Maintaining high power amplifier efficiency while maintaining signal linearity under back-off conditions can be addressed utilizing an envelope tracking type of amplifier. In an envelope tracking system, the drain supply voltage to the radio frequency power amplifier varies with respect to the average power contained in the input RF signal. Operationally, a Class-S modulator efficiently transfers the battery voltage to a different one supplied to the amplifier voltage. Alternatively, a linear regulator can be used in place of the Class-S modulator at the expense of lower overall efficiency. The RF power amplifier efficiency improves under back-off conditions because the DC power delivered to the amplifier is reduced from situations where the drain supply voltage equals the battery voltage. In addition, the amplifier remains linear as long as the drain supply voltage enables the amplifier to operate below gain compression and therefore does not significantly distort the amplitude of the modulated envelope.
A significant consideration in designing such an amplifier for high volume mobile radio handsets relates to establishing and implementing a control mechanism which yields the desired functional relationship between input power and the drain supply voltage. Present systems employ an open loop control where a control voltage determines the drain supply voltage developed by the Class-S modulator. This requires predetermined knowledge of the average power of the input signal and the desired functional relationship between the control voltage and the input power. A digital signal processor (DSP) typically determines the control voltage. However, the required functional relationship between the control voltage and the input power depends on the transfer characteristics of each individual component. The gain transfer characteristics of the RF power amplifier and Class-S modulator must be determined a priori. Such characteristics are set during manufacture and inherently include a significant margin, lowering the overall efficiency. Further, because the control system is open loop, the control system must compensate for component, parametric, temperature, and other variations and requires compensation through additional testing and characterization during radio phasing. Hence, component variations and the resulting cost, time, and complexity associated with compensating for variation is significant.
Many cellular systems have adopted time division multiple access (TDMA) architectures. In TDMA systems, the radio transmits and receives information only during certain specified time intervals. TDMA systems divide time into periods, defined as frames. The frames are further divided into intervals defined as slots. Frames repeat as time progresses. For example, a given mobile phone user, user A is typically assigned a slot by the base unit within the frame. User A may transmit voice and data information in that frame. User A transmits during the predetermined slot, and does not transmit during any other slot. User A continues to transmit in the predetermined slot of subsequent frames until directed otherwise. In systems utilizing TDMA architecture, it is desirable to maximize the trade-off between efficiency and linearity of the RF power amplifier. Because the amplifier transmits information only during specified time intervals, amplifier efficiency should be maximized during those time periods, with the signal exhibiting an acceptable level of linearity.
Mobile radios do not presently enable measuring the distortion level of the transmitted signal, and based on that level, adjusting the RF power amplifier to improve efficiency or linearity. This can be particularly advantageous in optimizing the trade-off between efficiency and linearity while the radio is in operation. Consider the following scenarios: 1) If the transmitted signal is slightly distorted, but falls within an allowable distortion level, present systems do not enable an adjustment to the RF amplifier to improve efficiency. An adjustment would increase distortion of the transmitted signal, but with less margin, while remaining within a required range. 2) If the transmitted signal is highly distorted beyond an acceptable level, present systems do not enable an adjustment to reduce signal distortion. Hence, the amplifier operates at an unacceptable distortion level.